Electronic structure and magneto-optical Kerr effect of Tm monochalcogenides

The optical and magneto-optical (MO) spectra of Tm monochalcogenides are investigated theoretically from first principles, using the fully relativistic Dirac linear combination of muffin-tin orbitals band structure method. The electronic structure is obtained with the local-spin-density approximation (LSDA), as well as with the so-called LSDA + U approach. In contrast to LSDA, where the stable solution in TmTe is a metal, the LSDA + U gave an insulating ground state. LSDA + U theory predicts the thulium ion in TmTe to be in an integer divalent state. It also shows a gradual decreasing of the energy gap with reducing of the lattice constant. LSDA + U theoretical calculations produce a similar energy band structure in TmS and TmSe, with twelve 4f bands fully occupied and hybridized with chalcogenide p states. The 14th f hole level was found to be completely unoccupied and well above the Fermi level and a hole 13th f level is partly occupied and pinned at the Fermi level. The occupation number of the 13th f level is equal to 0.12 and 0.27 in TmS and TmSe, respectively (valence 2.88+ and 2.73+). Such an energy band structure of thulium monochalcogenides describes well their measured bremsstrahlung isochromat spectroscopy (BIS), and x-ray and ultraviolet photoemission spectra as well as the optical and MO spectra. The origin of the Kerr rotation realized in the compounds is examined.